Production of titanium



United States Patent C 3,054,735 PRODUCTION OF TITANIUM Raymond L. Johnson, Palmer-ton, Pa., assignor to The New Jersey Zinc Company, New York, N.Y., a corporation of New Jersey N Drawing. Filed Mar. 29, 1960, Ser. No. 18,242 Claims. (Cl. 204-64) This invention relates to the production of metallic titanium and, more particularly, to a novel method of effecting electrodeposition of titanium metal from a fused salt bath.

An electrolytic operation which is particularly characterized by the production of relatively coarse electrodeposited metallic titanium is one wherein the titanium is deposited on that surface of a cathode which is distal with respect to an anode in direct bath-communication with the cathode. That is, a fused chloride salt bath containing titanium trichloride and titanium dichloride is electrolyzed between the anode and cathode while titanium tetrachloride is introduced into that portion of the bath which is in contact with the distal surface of the cathode and hence called the catholyte), the anode surface and the efiective distal cathode surface i.e. the surface of the titanium deposit) being in direct communication with one another through the bath uninterrupted by any physical barrier other than a pervious deposit of metallic titanium on the distal surface of the cathode which divides the bath into the aforesaid catholyte and a remaining portion called the anolyte. The electrolytic conditions are so maintained between the electrodes, and the anolyte and catholyte compositions are so maintained, that a porous deposit of metallic titanium is formed predominantly on the aforementioned distal surface of the cathode. Such an operation is described in the United States patents to Reimert and Fatzinger No. 2,848,397, Andrews No. 2,900,318 and Barnett No. 2,908,619.

In normal operation of an electrolytic cell according to the foregoing procedure, the electrolyzing conditions are so maintained at the outset as to build up on the distal surface of the cathode the desired porous metallic titanium deposit. Achievement of this condition is generally indicated by a value of the open circuit back electromotive force of the cell (hereinafter referred to as B.E.M.F.) within the range of about 2.4 to 3.2 volts when the concentration of dissolved titanium in the catholyte is at least 1%, and preferably about 2%, by weight. Continued operation of the cell is then generally controlled, by correlation of the rate of feed of titanium tetrachloride and the cell current as described in the aforesaid patents, so as to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity and coarse crystallinity of the metallic titanium deposit on the distal surface of the cathode.

I have now discovered that the output of the cell can be increased by a cyclic variation in the aforesaid operation in which, surprisingly, the introduction of titanium tetrachloride into the bath is discontinued during one phase of the cycle. Thus, the improvement in the operation of the aforesaid method pursuant to the present invention comprises continuing the introduction of titanium tetrachloride into the catholyte portion of the bath until the aforesaid concentration of titanium in solution is obtained, then discontinuing the introduction of the titanium tetrachloride into the bath while continuing the flow of electrolyzing current until the concentration of titanium in solution in the bath has been lowered to about A to 1% by weight, and thereafter resuming the introduction of titanium tetrachloride into the bath in a repetition of the foregoing cycle.

Patented Sept. 18, 1 962 The molten salt baths which are useful in practicing the invention comprise one or more of the halides of the alkali metals and alkaline earth metals. Thus, the chlorides, bromides, iodides and fluorides of sodium, potassium and lithium as well as the same halides of calcium, magnesium, barium and strontium may be used with advantage. However, in the interest of simplifying the recovery of the halogen which is liberated at the anode during electrolysis, I presently prefer to use only the chlorides of these metals. Although an individual halide may be used as a single constituent bath, I now prefer to use a combination of these halides inasmuch as such combinations are characterized by relatively lower melting points than the individual salts. It is particularly advantageous, when using a combination of the aforementioned halides, to mix these halides in proportions approximating a eutectic composition in order to obtain baths with low melting points. For example, I have used with particularly satisfactory results a eutectic mixture composed of 5 mol percent of sodium chloride, 40 mol percent of potassium chloride and 55 mol percent of lithium chloride, the resulting mixture having a melting point of about 350 C. Other useful eutectic mixtures are represented by the mixture composed of 48.5 mol percent of sodium chloride and 51.5 mol percent of calcium chloride having a melting point of 505 C. and by the mixture composed of 24 mol percent of barium chloride, 35 mol percent of sodium chloride and 41 mol percent of potassium chloride having a melting point of 552 C. Of course, as in all other molten salt electrolytic methods for the production of metallic titanium, the bath should be as completely anhydrous as possible and should be compounded of salts of high purity.

The titanium tetrachloride is advantageously supplied to the bath by introducing it directly into the molten bath either with or without a carrier gas such as argon. The cell atmosphere should, of course, be compartmented to maintain separation between the atmosphere above the portion of the bath into which the titanium tetrachloride is introduced and the portion of the bath from which the chlorine is evolved at the anode. Moreover, the cell is advantageously tightly closed in order to control the cell atmosphere. The cell electrodes should be constructed of material which will not introduce extraneous elements into the fused bath. Thus, a non-metallic anode such as graphite or carbon should be used, graphite having been found in practice to be wholly suitable for this purpose. Cathodes of nickel, and preferably of corrosion-resistant nickel base alloys, are useful in practicing the invention. At the prevailing cell temperature, the aforementioned cathode materials have been found not to contaminate the deposited metallic titanium to any significant degree.

The relative position between, and the arrangement of, the anode and cathode within the molten salt body should be such that (a) chlorine evolved at the anode will rise in the anolyte without entering the catholyte, (b) the anolyte and the catholyte are in communication with one another through a multiplicity of passages, and (c) the distance between the anode and the proximal cathode surface, and hence the resistance of the bath between these surfaces, is sufficiently small to permit electrolytically induced depletion of the titanium content of the molten bath between these surfaces.

A number of arrangements of anode and cathode will assure these conditions, and a variety of such arrangements is shown in the drawings in the aforementioned patent to Reimert and Fatzinger.

The electrolyzing condition which assures the maintenance of titanium-depletion in the anolyte, and deposition and maintenance of a porous and coarsely crystalline titanium deposit on the distal surface of the cathode, comprises the use of a voltage sufiiciently high to strip the anolyte of its titanium chloride content. When the anolyte is effectively stripped of its titanium chloride content, thus leavingessentially only the aforementioned molten bath composed of alkali metal or alkaline earth metal halides, the B.E.M.F. of the cell, measured across the anode and cathode upon opening of the exterior cell circuit, has a magnitude of about 2.6 volts or more when operating with a bath temperature of about 550 C. A B.E.M.F. below about 2.4 volts is an indication of the presence of titanium chloride in the anolyte portionr As the upper limit of about 3.3 volts is exceeded, and particularly as the B.E.M.F. reaches about 3.4 volts, decomposi tion of the non-titaniferous bath components such as the alkali metal chlorides begins to occur. It must be understood that, as appreciated by one skilled in the art of fused salt electrolysis, the maximum B.E.M.F. will be influenced by the titanium concentration in the catholyte, by the electrode compositions, by the bath temperature, and by the fact that the B.E.M.F. as measured is the average of the voltage between the anode and both the proximal and distal surfaces of the deposition cathode, but in general it can be stated that under most conditions the presently preferred upper limit for the B.E.M.F. is about 3.2 volts. The B.E.M.F. is maintained within the aforementioned range either by control of the cell voltage, so as to maintain appropriate depletion of titanium ions in the anolyte, or by controlling the rate at which the titanium tetrachloride is delivered to the cell for assimilation by the molten bath. Measurement of the B.E.M.F. at intervals of minutes is generally sufficiently frequent to permit the maintenance of a substantially uniform value at any desired level in the aforementioned range to within about one-tenth of a volt.

' The supply of titanium tetrachloride under cell electrolyzing conditions results in the build-up of the concentration of titanium in the solution. The dissolved titanium is in the form of titanium trichloride and titanium, dichloride. It is desirable that the concentration of dissolved titanium be sufficiently high to promote eifective deposition of metallic titanium on the distal surface of the cathode in the form of a porous and coarsely crystalline deposit. On the other hand, an excessive concentration of dissolved titanium tends to promote diffusion of this titanium into the anolyte portion of the bath and further promotes deposition of titanium within the pores of the cathode deposit with resulting diminution of the desired porosity of the deposit. It is presently preferred to establish during the build-up period a concentration of dissolved titanium of at least about 1% by weight and generally of at least about 2% by weight. A concentration of about 3% by weight, and even higher, can be used with advantage under proper control of other cell operating conditions including such variables as current density, cell geometry, cell size, effective cathode area, the average valence of the dissolved titanium, and the like.

After the aforementioned concentration has been estab lished, the titanium tetrachloride feed is discontinued pursuant to the present invention while nevertheless maintaining the flow of the electrolyzing current. Under these conditions, titanium metal is deposited on the distal surface of the cathode in a porous form. However, as the dissolved titanium concentration in the catholyte reaches a point within the range of about /4 to 1%, depending on the aforementioned variable cell operating conditions, the B.E.M.F. begins to rise. This rise in B.E.M.F. can be countered under these conditions only by lowering the electrolyzing current; butinasmuch as a lowering of this current, is accompanied by a corresponding lowering of the metal deposition rate, the rise of B.E.M.F. is used pursuant to the invention as an indication that this partial stripping phase should be discontinued and that the feeding oftitanium tetrachloride into the catholyte should be resumed while maintaining the same electrolyzing current d flow. It will be appreciated, accordingly, that both phases of the cyclic operation of my invention are operated at an electrolyzing current level which is conducive to maximum overall rate of titanium metal production.

The following operation is representative but not limitative of an electrolytic operation in which, after the desired titanium concentration has been established in the catholyte, the electrolysis is continued under substantially steadystate conditions of titanium tetrachloride feed and of electrolyzing conditions:

A cell such as that previously described was charged with approximately 3,000 lbs. of molten salt consisting of a eutectic mixture of KCl, LiCl and NaCl which melts at 350 C. The temperature of the melt was raised to 600 C. and this temperature was maintained through the run by external heating or cooling as required.

The operation was started by adding SOD-1,000 cc. of TiCl to the catholyte to act as a getter for any residual moisture in the melt and to furnish a small concentration of titanium before the application of current. The current was turned on and regular addition of titanium tetrachloride was made at a 2:1 ratio of tetrachloride feed to its electrochemical current equivalent. The current was raised as rapidly as the high limit of the B.E.M.F. permitted, and the high limit was maintained Within the range of 2.9 to 3.2 volts. The feed of titanium tetrachloride was continued until a dissolved titanium concentration of about 2% to 2.5% was reached. During this build-up period, the deposition cathode was continually accumulating a porous titanium deposit and the diffusion of melt from catholyte to anolyte was being continually reduced. The feed ratio was then reduced to about 1:1 and was maintained at this ratio until the desired amount of titanium had been added to the cell.

At the end of the steady-state operation, feeding of titanium tetrachloride was discontinued, and the cell was stripped of its dissolved titanium content and the metallic titanium on the deposition cathode was harvested.

The results of this type of operation not embodying the cyclic feature of my invention, and obtained in four independent, and similar but not identical, runs A, B, C and D are compiled hereinafter in the table.

The foregoing operation was then modified pursuant to the present invention by changing the steady-state phase to one in which no titanium tetrachloride was fed to the cell and in which the flow of electrolyzing current was continued until the dissolved titanium content of the catholyte had been reduced to between A and 1% by weight. The electrolyzing current input was controlled by the high limits of the B.E.M.F. which, for this part of the operation, were maintained at about 3.0-3.2 volts.

A second cycle build-up was then started in which the titanium tetrachloride feed to current ratio was maintained between 2:1 and 2.5 :1 and was continued untila dissolved titanium concentration of 2.5 to 3.0% was reached. The B.E.M.F. limits were the same as for the first cycle build-up phase.

A second cycle partial stripping phase was then undertaken until the final dissolved titanium concentration was reduced to between 0.25 and 1.0%.

The third cycle and succeeding build-up phases were the same as the second cycle build-up phase except that the titanium tetrachloride feed to current ratio was sometimes increased to 3:1. i

The third cycle and succeeding partial stripping phases were the same as the second cycle partial stripping phase except that in the final cycle the dissolved titanium concentration was reduced to substantially nil.

At the end of the final cycle stripping phase, the current was stopped and the melt was allowed to cool to 450-475 C. The cathode was removed from the cell and the deposit was removed by air-operated chisels.

The results of this type of cycle operation embodying the present invention, and obtained in five independent, and similar but not identical, runs E, F, G, H and I are r compiled, along with the results for runs A through D in the following table. In this table, the production capacity of the cell under the specified operating condi tions are related with reference to the capacity of the cell in run A for which the arbitrary unit of l is used.

It will be readily seen by comparing the results of steady-state runs A through D with the results of the cyclic runs E through I that the cyclic operation, although it involved a cessation of the feed of titanium tetrachloride during certain phases, nevertheless produced more metallic titanium of better quality with substantially the same total amount of current as in the steady-state type of operation.

I claim:

1. In the electrowinning of metallic titanium between an anode and a cathode in a fused halide salt bath containing titanium trichloride and titanium dichloride in solution wherein metallic titanium s deposted on the cathode and chlorine is evolved at the anode, the effective anode and cathode surfaces being in direct communication with one another through an anolyte portion of the bath uninterrupted by any physical barrier other than a pervious cathode deposit of metallic titanium on the cathode, the metallic titanium being deposited predominantly from a catholyte portion of the bath in contact with that surface of an initially perforate cathode which is distal with respect to the anode as a result of the flow of electrolyzing current through the perforate cathode and through a porous metallic titanium deposit thereon in its passage between the anode and the distal surface of the cathode while maintaining an open circuit back electromotive force of the cell within the range of about 2.4 to 3.2 volts, titanium tetrachloride being introduced into the catholyte portion of the bath in order to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity of the metallic titanium deposit on the distal surface of the cathode, the improvement which comprises continuing the introduction of titanium tetrachloride into the bath until said concentration of titanium in solution is obtained, then discontinuing the introduction of the titanium tetrachloride while continuing the flow of electrolyzing current until the concentration of titanium in solution in the bath has been lowered to about A to 1% by Weight, and thereafter resuming the introduction of titanium tetrachloride into the bath.

2. In the electrowinning of metallic titanium between an anode and a cathode in a fused halide salt bath containing titanium trichloride and titanium dichloride in solution wherein metallic titanium is deposited on the cathode and chlorine is evolved at the anode, the efiective anode and cathode surfaces being in direct communication with one another through an anolyte portion of the bath uninterrupted by any physical barrier other than a pervious cathode deposit of metallic titanium on the cathode, the metallic titanium being deposited predomi-.

nantly from a catholyte portion of the bath in contact with that surface of an initially perforate cathode which is distal with respect to the anode as a result of the flow of electrolyzing current through the perforate cathode and through a porous metallic titanium deposit thereon in its passage between the anode and the distal surface of the cathode while maintaining an open circuit back electromotive force of the cell within the range of about 2.4 to 3.2 volts, titanium tetrachloride being introduced into the catholyte portion of the bath in order to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity of the metallic titanium deposit on the distal surface of the cathode, the improvement which comprises continuing the introduction of titanium tetrachloride into the bath until a concentration of dissolved titanium of at least about 2% by weight is obtained, then discontinuing the introduction of the titanium tetrachloride while continuing the flow of electrolyzing current until the concentration of titanium in solution in the bath has been lowered to about A to 1% by weight, and thereafter resuming the introduction of titanium tetrachloride into the bath.

3. In the electrowinning of metallic titanium between an anode and a cathode in a fused halide salt bath containing titanium trichloride and titanium dichloride in solution wherein metallic titanium is deposited on the cathode and chlorine is evolved at the anode, the effective anode and cathode surfaces being in direct communication with one another through an anolyte portion of the bath uninterrupted by any physical barrier other than a pervious cathode deposit of metallic titanium on the cathode, the metallic titanium being deposited predominantly from a catholyte portion of the bath in contact with that surface of an initially perforate cathode which is distal with respect to the anode as a result of the flow of electrolyzing current through the perforate cathode and through a porous metallic titanium deposit thereon in its passage between the anode and the distal surface of the cathode while maintaining an open circuit back electromotive force of the cell within the range of about 2.4 to 3.2 volts, titanium tetrachloride being introduced into the catholyte portion of the bath in order to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity of the metallic titanium deposit on the distal surface of the cathode, the improvement which comprises continuing the introduction of titanium tetrachloride into the bath until said concentration of titanium in solution is obtained, then discontinuing the introduction of the titanium tetrachloride while continuing the flow of electrolyzing current until the concentra tion of titanium in solution in the bath has been lowered to about 4% by weight, and thereafter resuming the introduction of titanium tetrachloride into the bath.

4. In the electrowinning of metallic titanium between an anode and a cathode in a fused halide salt bath containing titanium trichloride and titanium dichloride in solution wherein metallic titanium is deposited on the cathode and chlorine is evolved at the anode, the effective anode and cathode surfaces being in direct communication with one another through an anolyte portion of the bath uninterrupted by any physical barrier other than a pervious cathode deposit of metallic titanium on the cathode, the metallic titanium being deposited predominantly from a catholyte portion of the bath in contact with that surface of an initially perforate cathode which is distal with respect to the anode as a result of the flow of electrolyzing current through the perforate cathode and through a porous metallic titanium deposit thereon in its passage between the anode and the distal surface of the cathode while maintaining an open circuit back electromotive force of the cell within the range of about 2.4 to 3.2 volts, titanium tetrachloride being introduced into the catholyte portion of the bath in order to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity of the metallic titanium deposit on the distalsurface of thecathode, and the assimilation of the titanium tetrachloride into the bath being aided by an auxiliary current flowing between the anode and. an auxiliary electrode which is positioned in the catholyte and which is cathodic with respect to the anode, the improvement which comprises continuing the introduction of titanium tetrachloride into the bath until said concentration of titanium in solution is obtained, then discontinuing the introduction of the titanium tetrachloride while continuing the flow of electrolyzing current until the. concentration of titanium in solution in the bath been lowered to about A to 1% by weight, and thereafter resuming the, introduction of titanium tetrachloride into the bath.

5 In the electrowinning of metallic titanium between an anode and a cathode in a fused halide salt bath containirig titanium trichloride and titanium dichloride in solution wherein metallic titanium is deposited on the cathode and chlorine is evolved at the anode, the effective anode and cathode surfaces being in direct communication with one another through an anolyte portion of the bath uninterrupted by any physical barrier other than a pervious cathode deposit of metallic titanium on the cathode, the metallic titanium being deposited predominantly from a catholyte portion of the bath in contact with that surface of an initially perforate cathode which is distal with respect to the anode as a result of the flow of electrolyzingcurrent through the perforate cathode and through a porous metallic titanium deposit thereon in its passage between the anode and the distal surface of the cathode while maintaining. an open circuit back electromotive force of the cell within the range of about 24 to 3.2 volts, titanium tetrachloride being in troduced into the catholyte portion of the bath in order to supply the titanium in solution therein in a substantially maximum concentration which permits the maintenance of the porosity of the metallic titanium deposit on the distal surface of the cathode, the improvement which comprises continuing the introduction of titanium tetrachloride into the bath until said concentration of titanium in solution is obtained, then discontinuing the introduction of the titanium tetrachloride while continuing the flow of electrolyzing current until the concentration of titanium in solution in the bathhas beenlowered to about A to 1% by weight with a back electromotive force not greater than about 3.2 volts between the anode and the deposition cathode, and thereafter resuming the introduction of titanium tetrachloride into the bath.

References Cited in the file of this patent UNITED STATES PATENTS 2,848,397 Reimert et a1 Aug. 19, 1958 

1. IN THE ELECTROWINNING OF METALLIC TITANIUM BETWEEN AN ANODE AND A CATHODE IS A FUSED HALIDE SALT BATH CONTAINING TITANIUM TRICHLORIDE AND TITANIUM DICHLORIDE IN SOLUTION WHEREIN METALLIC TITANIUM S DEPOSTED ON THE CATHODE AND CHLORINE IS EVOLVED AT THE ANODE, THE EFFECTIVE ANODE AND CATHODE SURFACES BEING IN DIRECT COMMUNICATION WITH ONE ANOTHER THEOUGH AN ANOLYTE PORTION OF THE BATH UNINTERRUPTED BY ANY PHYSICAL BARRIER OTHER THAN A PERVIOUS CATHODE DEPOSIT OF METALLIC TITANIUM ON THE CATHODE, THE METALLIC TITANIUM BEING DEPOSITED PREDOMINANTLY FROM A CATHOLYTE PORTION OF THE BATH IN CONTACT WITH THAT SURFACE OF AN INITIALLY PERFORATE CATHODE WHICH IS DISTAL WITH RESPECT TO THE ANODE AS A RESULT OF THE FLOW OF ELECTROLYZING CURRENT THROUGH THE PERFORATE CATHODE AND THROUGH A POROUS METALLIC TITANIUM DEPOSIT THEREON IN ITS PASSAGE BETWEEN THE ANODE AND THE DISTAL SURFACE OF THE CATHODE WHILE MAINTAINING AN OPEN CIRCUIT BACK ELECTROMOTIVE FORCE OF THE CELL WITHIN THE RANGE OF ABOUT 2.4 TO 3.2 VOLTS, TITANIUM TETRACHLORIDE BEING INTRODUCED INTO THE CATHOLYTE PORTION OF THE BATH IN ORDER TO SUPPLY THE TITANIUM IN SOLUTION THEREIN IN A SUBSTANTIALLY MAXIMUM CONCENTRATION WHICH PERMITS THE MAINTENANCE OF THE POROSITY OF THE METALLIC TITANIUM DEPOSIT ON THE DISTAL SURFACE OF THE CATHODE, THE IMPROVEMENT WHICH COMPRISES CONTINUING THE INTRODUCTION OF TITANIUM TETRACHLORIDE INTO THE BATH UNTIL SAID CONCENTRATION OF TITANIUM IN SOLUTION IS OBTAINED, THEN DISCONTINUING THE INTRODUCTION OF THE TITANIUM TETRACHLORIDE WHILE CONTINUING THE FLOW OF ELECTROLYZING CURRENT UNTIL THE CONCENTRATION OF TITANIUM IN SOLUTION IN THE BATH HAS BEEN LOWERED TO ABOUT 1/4 TO 1% BY WEIGHT, AND THEREAFTER RESUMING THE INTRODUCTION OF TITANIUM TETRACHLORIDE INTO THE BATH. 